The present invention relates to a light detection device for detecting invisible light, such as infrared light.
Infrared light, which has a longer wavelength than visible light, is used in optical communications. Since infrared light is invisible, one cannot visually identify the optical path of an infrared signal output from an end of an optical fiber cable.
Optical elements (e.g., diffraction gratings, mirrors, etc.) used in optical communications modules are inspected using infrared light. In detail, an optical element is fixed to a sample stage. Infrared light for inspection is output from one end of an optical fiber cable to irradiated the optical element. The optical element is evaluated based on reflected light from the optical element. To enable precise irradiation of the optical element with infrared light, the optical path position of the infrared light is first detected and then aligned with the optical element. The intensity of infrared light, which is output from an infrared light source, is generally maximum at the center of its optical path and becomes weaker at positions farther from the center. To irradiate the optical element with the strongest portion of the infrared light, the operator must detect the effective spot diameter of the infrared light including the center of its optical path.
To detect the effective spot diameter, infrared light detection devices, such as an infrared (IR) card, an infrared charge coupled device (CCD) camera, and an optical power meter, may be used.
The IR card has a surface on which a light-emitting area is formed by applying a light-emitting material. When subjected to infrared light, the light-emitting material emits visible light having an intensity according to the intensity of the infrared light. The operator visually identifies the visible light to detect the optical path of the infrared light.
The infrared CCD camera directly receives infrared light and displays the infrared light on its display unit. With the infrared CCD camera, the operator is able to detect the intensity distribution of infrared light with high sensitivity.
The optical power meter includes a sensor for receiving infrared light and a meter for displaying the intensity of the infrared light. The operator moves the sensor to receive infrared light with the maximum intensity so that the optical path of the infrared light can be identified.
Japanese Laid-Open Patent Publication No. 6-236574 describes a method for detecting an optical path position using an optical power meter. A sensor included in the optical power meter has four photo-detectors. When a two-channel oscilloscope receives an input of a first difference, which is the difference between the outputs of two photo-detectors into its X axis, and an input of a second difference, which is the difference between the outputs of the other two photo-detectors into its Y axis, the oscilloscope displays a bright spot on its CRT (cathode-ray tube). The operator identifies the optical path position based on the display position of the bright spot and adjusts the position of the sensor accordingly.
The IR card has low sensitivity and does not have a sensitization function. To enable the IR card to detect weak infrared light, the entire working environment needs to be darkened. In this case, the room in which inspections are performed needs to be darkened first for the operator to identify the optical path position of light. Then, the room needs to be lightened for the operator to adjust the position of the optical element. Thus, the setting of the working environment is burdensome. Further, since the IR card has low sensitivity, the operator cannot accurately identify the effective spot diameter. Moreover, the infrared CCD camera is relatively large and is not suitable for infrared light detection performed in a narrow space.
The optical power meter described in Japanese Laid-Open Patent Publication No. 6-236574, which is used to detect the optical path position, requires the operator to look at the CRT and adjust the position of the sensor. The operator needs to frequently look from one place to another. Thus, identification of the optical path position with this optical power meter takes a long time.
The IR card, the infrared CCD camera, and the optical power meter all require the optical element to be rearranged with respect to the detected optical path position after the optical path position is detected. Thus, precise irradiation of the optical element with infrared light using the IR card, the infrared CCD camera, and the optical power meter is difficult.